The demand for coniferous trees, such as pines and firs, to make wood products continues to increase. One proposed solution to meeting this high demand is to identify individual trees that possess desirable characteristics, such as a rapid rate of growth, and produce numerous, genetically identical, clones of the superior trees by somatic cloning.
Somatic cloning is the process of producing plant embryos, in vitro, from plant cells that are not zygotes. These clones can be cultivated to yield stands, or whole forests, of conifer trees that possess the desirable characteristic(s). One method for somatically cloning trees utilizes in vitro treatment of isolated, living, conifer tissue under conditions that promote formation of conifer somatic embryos, and then whole plants from the treated tissue. An explant such as an immature seed or the embryo from an immature seed is placed on a gelled initiation medium. This medium will usually contain plant growth hormones from the groups known as auxins and cytokinins. If initiation is successful a gelatinous mass containing multiple immature embryos will be generated in several weeks. This mass is then removed and subcultured on a maintenance and multiplication medium which may be gelled, liquid, or some combination of these. Embryos from maintenance medium may then be placed on a development medium that normally lacks the auxins and cytokinins but may instead include the hormone abscisic acid. The somatic embryos develop into cotyledonary embryos with a size and morphology that closely resembles their mature zygotic counterparts. The embryos are then placed on a germination medium on which radicle and epicotyl elongation occur over a period of several weeks. The germinants are then removed and placed in soil for further development into plantlets. After a period of greenhouse growth they may be outplanted. Alternatively, the embryos removed from the development medium may be placed into manufactured seeds; e.g., as shown in Carlson et al., U.S. Pat. No. 5,236,469.
A continuing problem, however, has been that conversion percentage from somatic embryos to plants growing in soil has frequently been lower than desired. In addition, a logistical problem exists for handling large numbers of conifer somatic embryos that are not packaged into manufactured seeds. These embryos are individually removed from the sterile development medium and placed on a germination medium. After an appropriate time the germinants are placed in a potting soil mixture for further growth. Typically many hundreds of thousands of germinants are potted at once in clonal field tests. The germinants are very tender and susceptible to damage by disease organisms such as fungi. In addition, it is preferable to outplant germinants that have similar dimensions in clonal field tests in order to obtain reliable results regarding the characteristics of various clones. However, a logistical challenge exists for producing a population of germinants having similar dimensions at the time of outplanting.
There is therefore a continuing need for a method for storing conifer germinants that reduces the risk of contamination with microbes, is amenable to large scale production, and results in a high conversion rate after transplantation.